Method for controlling the hydroxide ion concentration in pulp digestion liquor

ABSTRACT

A METHOD IS PROVIDED FORAUTOMATICALLY CONTROLLING THE HYDROXIDE CONCENTRATION OF PULP DIGESTER LIQUOR BY DETERMINING THE DIFFERENTIAL CONDUCTIVITY OF THE LIQUOR AT SPACED STAGES IN THE COOKING PROCESS AND ADJUSTING THE HYDROXIDE CONCENTRATION IN THE BATH FROM A PREVIOUSLY PREPARED CURVE OF C ONDUCTIVITY DIFFERENCES VERSUS HYDROXIDE ION CONCENTRATION.

Jan. 5, 1971 H. M. RIVERS METHOD FOR CONTROLLING THE HYDROXIDE ION CONCENTRATION IN PULP DIGESTION LIQUOR Filed April 1, 1968 ACTIVE ALKALI Differential Conducfivity (0;) l- I000 Active Alkali (In/n. as NcI O) Effective Alkali (lb/", us N0 0) INVENTOR Hubert M. Rivers .zf

United States Patent METHOD FOR CONTROLLING THE HYDROXIDE ION CONCENTRATION IN PULP DIGESTION LIQUOR Hubert M. Rivers, Upper St. Clair Township, Allegheny County, Pa., assignor to Calgon Corporation Filed Apr. 1, 1968, Ser. No. 717,564 Int. Cl. D21c 7/12 US. Cl. 16249 11 Claims ABSTRACT OF THE DISCLOSURE A method is provided for automatically controlling the the hydroxide concentration of pulp digester liquor by determining the differential conductivity of the liquor at spaced stages in the cooking process and adjusting the hydroxide concentration in the bath from a previously prepared curve of conductivity differences versus hydroxide ion concentration.

This invention relates to pulp digester controls and methods of pulp digesting and particularly to controls for alkaline pulp digesters for controlling the level of alkalinity throughout the cooking process.

It has long been recognized that alkaline digested wood pulp and the like material could be materially improved if the alkalinity in digester could be precisely controlled throughout all stages of the digestion process. Being comparatively slow and cumbersome, ordinary analytical methods are of limited utility in modern digester operation and, as a result, no entirely satisfactory method of control has been available for such equipment. At the present time, substantially all controls are centered at the beginning of the operation but the uncontrollable variables involved in the system are so great as to make this system unstable and not readily amenable to automatic controls. The operator is never able to ascertain at the beginning an accurate figure for moisture release, superficial moisture, softness of the wood, reactivity of the chips and even to have any real control of the initial chemical impregnation of the chips. All of these factors have a very marked effect on the cooking process, on the operation of the digester, and, consequently, on the quality of the finished product.

In order to have any effective control of digester operation, the operator must have some analytical means for rapidly and accurately monitoring the digestion process as it progresses in the digester. Up to the present time, no effective analytical means for accomplishing this objective has been available.

I have invented a method of pulp digester control and a control apparatus capable of providing the necessary monitoring of the cooking liquor and thus controlling the cooking process.

Preferably, I measure the conductivity of the cooking liquor, before and after neutralization, at strategic stages throughout the cooking processes. This may be done at some specified moment at the beginning or during the process or at spaced points along a continuous digester or at predetermined time intervals in a batch digester. Preferably, after determining the difference in conductivity before and after neutralization, which I shall hereafter call the differential conductivity, I then bring the differential conductivity to a previous determined desirable level by employing as may be appropriate or required such expedients as adding fresh chemical (e.g., white liquor) or dilution liquor (e.g., weak black liquor). As a further consequence of the measured differential conductivity value or values, the cooking process may be accelerated or decelerated by increasing or decreasing the temperature of the cooking liquor, or the duration of the cooking interval may be increased or decreased as required to satisfy product quality specifications. Preferably, I withdraw a sample of the cooking bath liquor at each of the spaced points or time intervals and measure its conductivity and thereafter I add a buffering-type reagent, such as carbon dioxide, capable of neutralizing certain alkaline constituents of the sample and again measure the conductivity of the reacted sample, determine the differential conductivity between the original and reacted sample and determine the amount of neutralized constituents present from a curve of differential conductivity versus concentration prepared from known concentrations of the same constituents.

The apparatus for use in my invention comprises a pulp digester, means for measuring conductivity of the solution in the pulp digester at spaced apart points representing different stages in the cooking process, means for adding a buffering agent to a sample of said solution, means for measuring conductivity of the sample after the addition of the buffering agent and means for determining the difference between the two conductivities to provide a differential conductivity measurement and, finally, means for adjusting the cooking liquor to bring the differential conductivity values to levels previously determined to be desirable.

In the foregoing general description of my invention I have set out certain objects, purposes and advantages of the invention. Other objects, purposes and advantages of this invention will be apparent from a consideration of the following description and the accompanying drawings in which:

FIG. 1 is a diagrammatic view of a pulp digester and control apparatus according to my invention;

FIG. 2 is a graph of differential conductivity versus active alkali and versus effective alkali in samples of cooking liquor from a digester such as that shown in FIG. 1.

Referring to FIG. 1, I have illustrated diagrammatically a combination pulp digester of the Kamyr type embodying my invention. Referring to the figure, I have shown a digester 10. Makeup liquor is supplied by a line 11 entering the digester. The digester is heated by steam supplied through a steam line 12. Chips are fed into digester 10 through line 13. Samples may be drawn at spaced apart points 14, 15 and 16 in the digester 10 by means of recirculation lines 17, 18 and 19. Each of the lines 17, 18 and 19 is connected by a line 20 to a heat exchanger 21 in which the sample is cooled to approximately room temperature. The cooled sample is then carried to a conductivity vessel 22 in which is placed a conductivity cell 23 where conductivity of the sample is first determined. The sample is then removed from the vessel 22 to a neutralizing chamber 24 Where CO gas is introduced through line 25 from a C0 tank 26 to neutralize the hydroxide ion (OH--) to bicarbonate ion (HCO The neutralized sample is then removed to a second conductivity vessel 27 carrying a conductivity cell 28 where the neutralized conductivity of the sample is measured. The conductivity cells are connected to conductivity bridges 29 and 30 of usual design and the conductivity of the liquids in each point is measured in the usual fashion and recorded on a two-pen recorder 31 of conventional design. The difference in conductivity or differential conductivity bears a direct relationship to the concentration of hydroxide as shown in FIG. 2.. This can be determined by a graph such as that shown in FIG. 2. Alternately, the signals from the twoconductivity bridges can be used to energize a signal system which produces a signal corresponding to differential conductivity which can, in turn, be used to regulate the addition of white liquor into the digester or the feed of the operation of the digester.

As an alternate practice of the invention using a single conductivity cell on each of the lines 17, 18 19, the following sequence of operation is used:

(1) fill the conductive cell with sample; (2) measure the initial conductivity C (3) record the initial conductivity C and (4) introduce CO as required for complete neutralization of the sample; (5 measure the neutralized conductivity C (6) record the neutralized conductivity C (7) compute C -C the differential conductivity;

(8) record C -C or alternately generate a related conductivity signal; and

(9) discharge the sample to make way for the next cycle of sampling.

The differential conductivity for digester liquor samples was determined for a pulp digester at various points in the digester. The test results are tabulated in Table This invention may also be practiced by simultaneously drawing two samples from the bath of liquor, measuring the conductivity of one sample, neutralizing and thereafter measuring the conductivity of the other sample, determining the difference and using the difference as the differential conductivity. This is desirable in some instances in order to speed up the final determination. Similarly, the conductivity may be measured at one point and the neutralized conductivity at a point spaced therefrom.

While I have illustrated and described certain present typical practices and embodiments of my invention, it will be understood that this invention may be otherwise embodied within the scope of the following claims.

I claim:

1. The method of controlling the hydroxide concentration of a pulp digester liquor comprising the steps of measuring the conductivity of the cooking bath in the digester at at least one point in the cooking process, measuring the neutralized conductivity at the same point, determining the dilference in conductivity between the two said measured conductivities, and adjusting the hydroxide concentration in the bath from a curve of the conductivity difference versus hydroxide ion concentration pre- TABLE I.CHANGE IN CONDUCTIVITY oNglilg IlliTRALIzATlON OF DIGES'I'ER LIQUOR SAMPLES Plants Active Sample Date of Conductivity, micromhos/cm. effective alkali Date of conductivity alkali value No./cu.tt. Location No. sampling tests Initial Final Difference (lo/ft!) as NagO Top circulation pump 1 143-67 126-67 264,000 98, 800 165,200 3. 61 4. 39 Do 2 11367 1-26 67 256, 000 9! 800 158, 200 3. 63 4. 39 3 116 67 1-27-67 274, 000 98, 900 176, 000 3. 88 4. 59 4 1-16-67 1-27-67 274, 000 99, 800 174, 200 3. 89 4. 61 5 11767 13(Hi7 274, 000 98, 800 175,200 3. 81 4. 49 6 1-17-67 1-30-67 274, 000 98, 600 175,400 3. 89 4. 57 1 1-13-67 1-26-67 120, 000 70,600 49, 400 1. 26 1. 74 2 1-13-67 1-26-67 119, 600 72, 400 47,200 1. 1. 92 3 1-16 67 12767 133, 600 73, 800 59, 800 1. 50 1. 92 4 1-16-67 127-67 122,400 71,600 50, 800 1. 36 1. 84 5 1-17-67 130 67 123,200 71, 800 51, 400 1. 35 l. 81 6 1-17-67 1-30-67 122, 000 71, 409 50, 600 1. 32 l. 78 1 1-13-67 1-2667 99, 200 66, 800 32, 400 1. ()5 1. 46 2 l-13-67 1-26-67 100, 800 67, 400 33, 400 1. 05 1. 46 3 1-16-67 1-27-67 98, 400 66, 600 31, 800 1. 05 1. 46 4 1-16-67 1-27-67 102, 600 67 000 35, 600 1. 12 1. 5 1-17-67 1-30-67 100, 800 67, 400 33, 400 1. 09 1. 50 Do 6 1-17-67 1-30-67 103, 200 68, 400 34, 800 1. 07 1. 49 Upper wash extraction.... 1 143 67 1-26-67 71, 000 58,400 12,600 0. 56 0. 92 Do 2 1-13-67 1-26-67 67, 400 58,200 9, 200 0. 59 0. 93 3 1-16-67 1-27-67 70, 000 59,400 10, 600 0. 59 0. 93 4 1-16-67 1-27-67 69, 000 59,400 9, 600 0. 0. 94 5 1-17-67 1-30-67 72, 000 59, 800 12, 200 0. 60 0. 94 6 1-17-67 1-30-67 72, 800 59, 400 13,400 0. 64 0. 97

The ditferentlal COIldllCllVl'tY versus act1ve alkali and pared from known concentrations of alkaline constituents effective alkali for the samples shown in Table I are graphed in FIG. 2 of this application.

In the foregoing examples, I have attempted to show the effectiveness of my method of pulp digester control and the manner in which it may be effectively used in the control of pulp digester liquor composition so as to provide a uniformity in digester operation and product quality which has heretofore been impossible. I have in these examples used carbon dioxide as the buffering agent. Other appropriate buffering agents, such as acetic acid, boric acid, gallic acid, citric acid and salicylic acid, for

example, may be used.

A batch digester may be controlled in essentially samples are removed at spaced periods of time and their 1 differential conductivity determined precisely as in the case of the continuous digester. Adjustments to the cooking liquor are made by adding dilution water, white liquor or black liquor, or by changing the heat input precisely as mentioned above.

and adding said given constituents to the bath to bring the level of hydroxide concentration in the bath to a desired point.

2. The method as claimed in claim 1 including with drawing a sample from the cooking bath, measuring the conductivity of said sample, adding a buffering agent to said sample, measuring the conductivity of the buffered sample, determining the difference between the two conductivities so measured, determining the hydroxide concentration from a curve of conductivity difference versus hydroxide ion concentration.

3. The method as claimed in claim 1 including the steps of simultaneously drawing two samples from the bath of liquor, measuring the conductivity of one sample, neutralizing and thereafter measuring the conductivity of the other sample, determining the difference between the two said measured conductivities and using said difference as the differential conductivity.

4. The method as claimed in claim 2 wherein the buffering agent is carbon dioxide.

5. The method as claimed in claim 3 wherein the two conductivity measurements are made in separate conductivity cells, each delivering an electrical signal commen surate with the conductivity value at the cell, supplying said signals to a pair of conductivity bridges supplying an output signal and regulating the flow of white liquor into the digester through a control system responsive to said output signal.

6. A method as claimed in claim 5 wherein the control system responsive to said output signal regulates the flow of black liquor into the digester.

7. A method as claimed in claim 5 wherein the control system responsive to said output signal regulates the flow of steam to the digester.

8. A method as claimed in claim 5 wherein the control system responsive to said output signal regulates the flow of dilution water to the digester.

9. The method as claimed in claim 1 wherein the conductivity is measured at spaced apart stages in the cooking process and the neutralized conductivity is measured at the same said spaced apart stages in the cooking process.

10. The method of controlling the hydroxide concentration of a pulp digester liquor as claimed in claim 1 wherein the conductivity of the cooking bath in the digester is measured at one point and the neutralized conductivity of the digester is measured at a point spaced from said one point to control the hydroxide ion concentration.

11. The method of controlling the hydroxide concentration of a bath of pulp digester liquor comprising the steps of measuring the conductivity of the bath of liquor in the digester at a point in the cooking process, neutralizing the portion of the bath in which the conductivity has been measured, measuring the neutralized conductivity of the neutralized portion of the bath, determining the differential conductivity between the original conductivity and the neutralized conductivity, establishing a control condition at the sampling point from the differential conductivity and altering the condition of the bath in response to the differential conductivity to move the differential conductivity to a desired point previously determined.

References Cited UNITED STATES PATENTS 4/1949 Wells 16249 3/1954 Westcott 162-62 US. Cl. X.R. l6262, 242 

